This invention relates generally to injection molding and more particularly to a valve gated hot runner injection molding system having a pneumatically driven rack and pinion valve pin actuating mechanism.
Valve gated injection molding systems are well known in the art, as are the related problems of valve pin actuating mechanisms. Actuating mechanisms are usually either lever operated or piston operated. A lever operated mechanism is shown in the applicant's U.S. Pat. No. 4,222,733 which issued Sept. 16, 1980. While this arrangement is satisfactory for many applications, it has the disadvantage that the lever unavoidably applied a lateral force to the reciprocating valve pin which causes friction, uneven wear and a larger gap on one side than the other. As a result, during the operating life of the system there is an uneven buildup of melt deposits around the valve pin which can cause leakage and malfunction. Other examples of lever operated valve pin mechanisms having similar problems are shown in the following brochures; Incoe "SVG 5000, SVG 7000", H. Muller Mekaniska AB "Flytgot Typ N" and "Precision Products GmbH Delta" Spritzduse Mit Hydraulicverschluss.
Actuating mechanisms having a piston connected directly to the valve pin have been used to overcome these problems associated with lever operated mechanisms. Examples of such piston operated mechanisms are shown in U.S. Pat. Nos. 4,380,426 to Wiles which issued Apr. 19, 1983, 4,433,969 to the applicant which issued Feb. 24, 1984 and 4,755,131 to Schmidt which issued July 5, 1988. While these piston operated systems do reduce the lateral forces applied to the valve pin they have the disadvantage that they are relatively costly to make and require a considerable amount of space around the valve pin head to which the piston is connected. This is particularly a problem in providing for the melt passage in a center entry single nozzle system as described in U.S. Pat. No. 4,380,426 referred to above. Also, many of the piston operated systems are hydraulically driven which has the additional problems of hot seals and continuing to circulate the oil for cooling after shut down. Furthermore, in a stack molding configuration the additional height required for the actuating mechanism is a considerable disadvantage.